Clostridium difficile is the most common cause of infectious diarrhea in hospital patients and is associated with substantial morbidity, mortality and financial cost. The longterm goal of this project is to assist in the development and clinical application of novel non-antibiotic agents to prevent and treat C. difficile-associated diarrhea and colitis. The central hypothesis of this proposal is that host factors, especially the immune response to C. difficile antigens, play a major role in determining the clinical outcome of C. difficile infection. We will perform a prospective cohort study of 150 subjects with C. difficile-associated diarrhea (CDAD) to examine each of the following three hypotheses: Hypothesis 1: lmmune responses to both C. difficile toxin A and toxin B are instrumental in protecting against recurrent CDAD. Our recent studies have shown that a serum IgG response to C. difficile toxin A is evident in subjects that are protected against symptomatic or recurrent CDAD. In this aim we will determine whether humoral and cell-mediated immune responses to toxin B, immune responses to toxin A recombinant peptides and/or toxin neutralizing activity are also associated with protection. Hypothesis 2: Risk for recurrent CDAD can be predicted based on clinical parameters and or anti-toxin antibody measurement. Based on our previous study of patients with CDAD (derivation cohort) we have developed prediction rules with >80% accuracy in predicting recurrence. In this aim we will prospectively test these rules in the 150 subjects recruited for Specific Aim 1 (validation cohort). Once validated these tools can be used in clinical practice and in research studies to identify high-risk patients that are most likely to benefit from measures to prevent recurrent C. difficile infection. Hypothesis 3: lmmune responses to C. difficile Surface Layer Protein (SLP) can protect against colonization and/or CDAD. Previous studies of the immune response to C. difficile have focused almost exclusively on anti-toxin antibodies. C. difficile surface layer protein (SLP) is the predominant surface protein in C. difficile, has been characterized recently at the molecular level and appear to act as a bacterial adhesin. Our preliminary data indicate that an immune response to C. difficile surface layer protein may be protective. In this aim we will compare humoral and cell-mediated immune responses to purified and recombinant C. difficile SLP in subjects with a single episode of CDAD, those with recurrent disease and in disease and healthy controls. We will also determine whether vaccination against C. difficile SLP can prevent colonization by C. difficile and/or C. difficile associated ileo-cecitis in hamsters. These specific aims are designed to advance further our knowledge of the immunobiology of C. difficile toxin-induced diarrhea and the mechanisms of immune protection in humans. [unreadable] [unreadable] [unreadable]